Polymeric foam materials include a plurality of voids, also called cells, distributed within a polymer matrix. Microcellular foams (or microcellular materials) are a type of polymeric foam characterized by having small cell sizes and high cell densities. Microcellular foams may have a number of property and processing advantages.
Certain processing steps may be required (or preferred), when forming microcellular foams. The steps include introducing blowing agent into polymeric material within an extruder, sufficiently mixing the mixture of polymeric material and blowing agent to form a homogeneous mixture within the extruder, maintaining a high pressure within the homogeneous mixture to prevent premature nucleation of the microcells, and rapidly reducing the pressure to nucleate the microcells at a desired time.
Conventional injection molding or blow molding systems generally are not capable of satisfying all of the aforementioned steps needed to produce microcellular materials. Typically, conventional systems are designed with the intention of operating under a set of conditions inconsistent with the production of microcellular material. Consequently, new systems have been specifically designed and manufactured to produce microcellular materials. New injection molding or blow molding systems, for example, have been designed with screws that have relatively high L:D ratios (e.g., L:D ratios of 28:1 and greater). In particular, screws having these relatively high L:D ratios can enable sufficient mixing of the polymeric material and blowing agent mixture and maintenance of high pressure within the mixture, both of which promote formation of microcellular material. However, screws having high L:D ratios may be incompatible with existing conventional equipment used in injection and blow molding systems. Furthermore, it may be more expensive to produce new or retrofitted systems that include screws having high L:D ratios.